It is well known that the surface treatment of various kinds of plastic film by exposure to a high voltage corona discharge improves the adhesion characteristics of the film surface so as to render it receptive to adherends such as printing ink.
The typical corona discharge treating apparatus for this purpose includes a stationary electrode connected to a high voltage alternating current source and positioned close to a backing roller about which the flim is passed and which serves as the ground electrode.
In existing apparatus the roller electrode comprises a steel or aluminum core, either solid or tubular, coated with a dielectric material that serves as an insulative barrier between the core and the corona discharge. The core is ground and polished to a very accurate dimensional tolerance preparatory to receiving the dielectric coating whose outer surface must also be accurately ground. Such rollers are both expensive to manufacture and to recoat. Recoating is necessary whenever the dielectric barrier breaks down causing arcing between the electrodes and consequent damage to the film being processed. Such breakdowns occur frequently.